Method, device and software for the optical inspection of a semi-conductor substrate

ABSTRACT

The invention relates to a method and a device for the optical inspection of the surface of semi-conductor substrate. An image ( 1 ) is captured on the surface of the semi-conductor substrate which is covered with a thin layer. Said image is made of a plurality of pixels having associated colour values and intensities. The frequency distribution of pixels having equal colour co-ordination values is calculated ( 3,4,5 ) from the colour values in a colour range ( 2 ), said colour range having a colour intensity and colour co-ordinates. The thus calculated frequency distribution is used ( 7, 9 ) to compare a second correspondingly calculated frequency distribution or a variable derived therefrom. According to the invention, the colour shift ( 9 ) and/or differences ( 7 ) in the colour distribution are determined according to fluctuations in the intensity of the illumination. The invention also relates to a method and a device for producing a structured semi-conductor substrate by using the above-mentioned method or the above-mentioned device and software for carrying out said method.

The invention relates to a method, a device, and a software for theoptical inspection of the surface of a semiconductor substrate, as wellas to a method and a device to produce a structured semiconductorsubstrate that uses such a method and/or such a device.

Semiconductor components are generally produced in a multiplicity ofprocessing steps. Thin layers are repeatedly applied to the surface of asemiconductor substrate, for example, photoresist layers, thinmetallized coatings, or dielectric layers. To achieve a high level ofconsistent quality, the thin layers must be applied at a consistentquality. In modern semiconductor technology, a plurality ofsemiconductor components are generally produced on a single wafer.Individual wafer substrates are illuminated with the help of a stepper,and the illumination step is repeated several times. Consistent qualityof illumination requires a homogeneous surface covering, particularly inthe application of a homogeneous photoresist layer on the wafer. Forthis reason, it is desirable to have a simple means of assessing qualityof thin layers on a semiconductor substrate, in particular theirhomogeneity.

Because of interference effects, thin layers reflect light with color.As a result, if the surface of a semiconductor substrate with thinlayers is illuminated and the reflected light analyzed, inhomogeneitiesin the thickness of the layer can be detected as color variances.

A method is known from the state of the art for using macroscopic imagesof the surface of a semiconductor substrate covered with thin layers todetermine mean values and color dispersions, and to compare these witheach other and with a reference wafer. Fluctuations in the brightness ofthe illumination, however, lead to fluctuations in the brightness of thereflected light so that the mean values for colors in the RGB range arein principle dependent on brightness. In addition, mean values anddispersion describe only a portion of the color changes that occur.

The task of the present invention is to create a method, a softwareprogram, and a device for the optical inspection of the surface of asemiconductor substrate in order to determine even more reliablyprocessing defects in the application of thin layers to a semiconductorsubstrate. In addition, a method and a device to produce a structuredsemiconductor substrate are to be created that permit the production ofsemiconductor components of consistently high quality.

This task is solved by a method with the characteristics according toclaim 1 and 13 or 14, respectively, by a software program according toclaim 28 for implementing the method, as well as by a device with thecharacteristics according to claim 15 and 26 or 27, respectively. Otheradvantageous embodiments of the invention are the subject of the relatedsubordinate claims.

According to the invention, to optically inspect the surface of asemiconductor substrate to which a thin layer has been applied, an imagethat is made up of a plurality of pixels is captured of the surface ofthe semiconductor substrate, each with at least three associatedintensities of varying wavelength that are designated as color values; afrequency distribution of pixels with the same color coordinate valuesis calculated from the color values by transformation in a color rangespanning one intensity and color coordinates; and the frequencydistribution calculated therefrom is used for comparison with a secondcalculated frequency distribution or with a variable derived therefrom.

The advantage is that the result of the comparison is independent of theintensity of the light reflected from the surface of the semiconductorsubstrate, and therewith independent of fluctuations in theillumination, because the intensity of the pixels does not have to beconsidered when calculating the frequency distributions. According tothe invention, processing defects in applying thin layers to asemiconductor substrate can therefore be determined even more reliably.As a result of processing defects, inhomogeneities in the thickness ofthe layers may, for example, occur, or layers may be missing. Processingdefects may also occur as a result of defocusing during illumination ofthe semiconductor substrate. In particular, the intensity of theillumination used to capture images can be changed, according to theinvention, in order, for example, to effect resolution of variousdefects on the surface of the semiconductor substrate without having itsignificantly affect the calculated frequency distribution. Variousangles of incidence may be used for illumination, in so far as the angleis the same as when teaching a reference wafer.

It is particularly preferable that three color values, i.e., intensitiesof varying wavelengths with the color range spanning one intensity andtwo color coordinates be associated with each pixel. However, this isnot a limitation on the present invention. Rather, the present inventioncan in principle be applied to ranges with higher dimensions, not merelyto the three-dimensional range. For example, four color values can alsobe associated with the pixels.

To capture images, a conventional color imaging sensor of suitablespectral sensitivity such as a color camera, video camera, CCD sensor,or one-dimensional color line scanner can be used to create a digitalimage composed of pixels with associated color values. Very preferably,RGB components spanning a three-dimensional range are associated witheach pixel.

Intensities with three other wavelengths that lie completely orpartially in the ultraviolet and/or infrared wavelength range of lightcan be used in place of the RGB components. “Color” imaging sensors ofsuitable spectral sensitivity are used for that purpose.

Depending on the specific requirements, the color imaging sensor is usedto capture a macroscopic image of the entire surface of thesemiconductor substrate or of a suitable subarea thereof. For thispurpose, the imaging range of the color imaging sensor can also bechanged depending, for example, on the requirements of each comparisonbeing conducted. The color imaging sensor can be coupled with a suitableimaging device such as a microscope.

The second frequency distribution used for the comparison can, forexample, be calculated and stored on the basis of at least one image ofa reference wafer of satisfactory quality. The second frequencydistribution can also be calculated, for example, on the basis of atleast one image of a second wafer from a current processing batch orportion thereof. In addition, the second frequency distribution can alsobe calculated on the basis of at least one image of a range of thecurrent wafer that is subject to inspection. It is also possible tocompare the frequency distribution of the reference wafer images withthe current wafer, in which case local and global color variances can bedetermined simultaneously. It is advantageous that the method accordingto the invention can be very flexibly adapted to the specificrequirements of the comparison being conducted.

Preferably, the RGB components of the pixel color values are transformedin the color range by means of linear transformation, which saves oncalculation time. In addition, color deviations or color shifts derivedfrom the comparison can be better compared with each other as a result.Preferably, the frequency distribution and/or the second frequencydistribution used for the comparison is calculated by totaling thefrequency of the occurrence of pixels with the same color coordinatevalues in the color range. Preferably, the frequency distributioncorresponds to a two-dimensional histogram in the color range used.

According to a particularly preferred embodiment of the invention, thecolor range used is a YUV color range, whereby Y corresponds to thelight intensity or luminescence of the pixels, and Y itself is notconsidered when calculating the frequency distribution. It isadvantageous that the YUV color range is used in the state of the art tocode color images and color videos. Efficient and inexpensive chips areavailable for video processing as well as for image compression hardwarefor the method according to the invention.

However, the present invention is in principle not limited to the use ofa YUV color range. Rather, other color ranges known from the state ofthe art can be used, such as the YIQ color range, YCbCr color range, orthe like. However, linear transformation of the pixel color values inthe color range is particularly preferred for effecting transformation.

If the color values for the colors red (R), green (G), and blue (B) arenot detected in the RGB color range, but in other spectral regions,comparable mathematical transformations can be used to transform thecolor values in the color range being evaluated, according to theinvention.

Preferably, the frequency distribution calculated in the color range andused for the comparison is smoothed by using a filter such as a boxfilter. It is advantageous that the resultant noise elicited byfluctuations in the frequency distribution be suppressed or at leastlessened, which increases the precision of the method still further.

As is well known, a plurality of semiconductor components or dies isapplied to a wafer in the engineering of semiconductors. According tothe invention, and depending on the specific requirements, it ispreferable according to the invention that the frequency distribution bealternatively calculated on the basis of images captured of at least onesemiconductor substrate comprising a plurality of semiconductorcomponents or dies, or of a surface area illuminated in a stepperillumination step (stepper area window; SAW) of the semiconductorsubstrate; or of an individual die; or of a subarea of a die. The areasused for capturing images can preferably be combined in any manner tocalculate the first and the second frequency distribution. For example,the first frequency distribution may be calculated on the basis of animage of a single die or of a SAW, while the second frequencydistribution is calculated on the basis of an image of the entiresurface of a reference wafer. This permits one to compare the quality ofindividual semiconductor components with each other on one and the samewafer; for example, of adjacent semiconductor components or of selectedcomponents on the substrate or wafer.

According to a first embodiment of the invention, a center of gravity iscalculated for the comparison from the calculated frequencydistributions, and the position of the center of gravity is comparedwith the position of the center of gravity of the second frequencydistribution in order to detect a color shift from which conclusions maybe drawn, for example, regarding systematic fluctuations in thickness ofthin layers on the same semiconductor substrate. The second frequencydistribution can be calculated on the basis of an image of a referencewafer or, for example, of an adjacent semiconductor component on thesurface of the same semiconductor substrate. The centers of gravity inthe color range used represent simple coordinate values that can easilybe compared with one another. As a result, color shifts can be detectedreliably with a high level of precision.

According to a second embodiment of the invention, the calculatedfrequency distribution is subtracted from the second frequencydistribution to make the comparison in order to detect differences incolor distribution for the semiconductor substrate. The second frequencydistribution can be calculated on the basis of an image of a referencewafer or, for example, of an adjacent semiconductor component on thesurface of the same semiconductor substrate. It is advantageous thatdifferences in the frequency distributions can be more easily detectedand resolved by generating difference. In order to bring out suchdifferences, the difference can, for example, be further intensified bymultiplication by a predetermined factor.

Obviously, the first and second embodiments of the invention may becombined with each other.

Furthermore, an alarm signal, a variable, or the like can be createdwhen a detected color shift according to the first embodiment and/or thedifferences detected in color distribution of the semiconductorsubstrate according to the second embodiment exceed a predeterminedthreshold value. The alarm signal, the generated variable, or the likecan thus be enlisted to automatically interpret the inspection process,for example, in a device that produces a structured semiconductorsubstrate, such as is used on semiconductor production lines that arewell known from the state of the art.

According to a further embodiment of the invention, the threshold valueused may be calculated by averaging the frequency distributions ofsurface areas that are geometrically arranged in a given configurationon a wafer. A radial distribution of the surface areas used isparticularly preferable. In this way, effects that lead toradially-dependent heterogeneous layer thickness on a wafer, for exampleas a result of spin coating a photoresist layer, can be easily takeninto account.

Obviously, any i moments of the calculated frequency distributions canbe used for the comparison, whereby i is a whole number and i≧1.

To implement the method according to the invention, a device for theoptical inspection of the surface of a semiconductor substrate comprisesa suitable imaging sensor of suitable spectral sensitivity such as acolor camera, video camera, or CCD sensor; suitable computationaldevices such as microprocessors, special processors, or the like; andsuitable means of comparison such as microprocessors, specialtyprocessors, or the like.

According to a further embodiment of the invention, the method accordingto the invention can be reduced to practice with the help of softwareand/or a computer program that comprises a program code in order toimplement the steps of the method according to the invention when thesoftware or the computer program is implemented in a computer or othersuitable data processing mechanism to control the computational deviceand means of comparison. Preferably, the software and/or the computerprogram comprises a program code that can be stored on a storage mediumthat can be read by a computer.

An example of a preferred embodiment of the invention is described belowon the basis of the appended figures, wherein:

FIG. 1 depicts a schematic block diagram of a device to implement themethod according to the invention;

FIG. 2 depicts the spectral sensitivity of a sensor camera that detectslight at three different wavelengths in the visible spectral region;

FIG. 3 depicts a two-dimensional histogram in a YUV color range that iscalculated with the help of the device depicted in FIG. 1;

FIG. 4 depicts the two-dimensional histogram according to FIG. 3 aftersmoothing;

FIG. 5 depicts the two-dimensional histogram according to FIG. 3 that isoverlaid with a second histogram whose of center of gravity is identicalto the center of gravity of the two-dimensional histogram according toFIG. 3; and

FIG. 6 depicts a schematic of an arrangement consisting of onesemiconductor substrate and a camera.

The method according to the invention serves to optically inspect thesurface of a semiconductor substrate 21, for example, a semiconductorwafer whose surface is covered with one or several thin layers such as aphotoresist layer, a metallization, a dielectric layer, or the like.Interference effects impart to the surface a color that is dependent onthe thickness of the thin layer. As a result, the light reflected fromthe surface permits one to draw conclusions regarding the thickness ofthe thin layer. Color variances in the reflected light intensity permitone to draw conclusions regarding inhomogeneities in the thickness ofthe thin layer.

The surface of the semiconductor substrate 21 or a portion of thesurface, such as a so-called stepper area window (henceforth SAW) thatcomprises a plurality of dies or semiconductor components, or an areawith one or several dies, or a part of a die, can be macroscopicallycaptured with the help of a CCD camera 1 that serves as a color imagingsensor. The image information is comprised of a plurality of pixels withassociated color values and intensities.

FIG. 2 represents the spectral sensitivity of a sensor camera 1 thatdetects light at three different wavelengths in the visible spectralregion, namely for the colors red, green, and blue. The color camera 1provides three intensity values for each pixel in the color image. Thevalue of each channel is dependent on the spectral sensitivity of theindividual sensors and on the incident light. FIG. 2 shows the spectralsensitivity of a 3-sensor camera 1, whose sensors A, B, and C aresensitive to visible light.

If φ_(λ) is the spectral distribution of the incident light, sensors A,B, and C provide the following intensities:I_(A) = k ⋅ ∫_(o)^(∞)a(λ)  ϕ_(λ)(λ)  𝕕λI_(B) = k ⋅ ∫_(o)^(∞)b(λ)  ϕ_(λ)(λ)  𝕕λI_(C) = k ⋅ ∫_(o)^(∞)c(λ)  ϕ_(λ)(λ)  𝕕λwhereby k is an amplification factor.

The image information is entered into an image processing device thattransforms the RGB components of the image information into the YUVcolor range.

The YUV color range is the basis for color coding in the televisionnorms used in Europe and is known to be comprised of the RGB componentsof the image information as follows:Y=0.299R+0.587G+0.114BU=−147R−0.289G+0.437B=0.493 (B−Y)V=0.615R−0.515G−0.100B=0.877 (R−Y)

The Y components represent luminescence. The YUV color range thus spansthe intensities and the color coordinates U, V.

The Y component is not considered during further processing of the imageinformation, which is indicated in FIG. 1 by the incomplete arrowbetween blocks 2 and 3. The remaining U and V color coordinate valuesspan a two-dimensional range of color. The frequency of the occurrenceof a pixel with the same U and V values for each captured image area istotaled with the help of a computational device 3. Thus the frequencydistribution 12 (histogram) depicted in FIG. 3 is calculated in thetwo-dimensional color range. The frequency distribution 12 exhibits twopeaks 13, 14 as well as one subordinate peak 15 that is attributable toimage artifact.

Because digital image processing is capable of only limited resolution,the calculated frequency distribution 12 exhibits discrete steps.Depending on the resolution used, for example 8-bit, the calculatedfrequency distribution 12 is overlaid with a discrete noise that caninterfere with subsequent comparisons and that leads to the double peaks13, 14 in FIG. 3.

The calculated frequency distribution 12 is smoothed with the help of afilter 4, for example with the help of a box filter. Suitablealternative filter algorithms will be clear to a person skilled in theart examining this description, and requires thus no furtherexplanation.

FIG. 4 shows schematically the smoothed frequency distribution 12according to FIG. 3. Depending on the width of the set frequency windowof the filter 4, a small amount of negligible spreading of the frequencydistribution 12 will occur such that double peaks 13, 14 converge into asingle peak 16 in FIG. 4.

The center of gravity of the calculated frequency distribution 12,expressed in U and V color coordinates, can be determined with the helpof the other computational device 5. In the color range spanned by the Uand V color coordinates, each position corresponds to a color of thereflected light intensity. If the light is reflected as many colors, thesmoothed frequency distribution 12 exhibits more than one peak.

The subsequent processing steps of the method according to the inventionare depicted schematically in the lower part of FIG. 1. For thecomparison step carried out alternatively in block 7 or 9, the methoduses a second frequency distribution (reference 2-dimensionalhistogram). The second frequency distribution is calculated on the basisof light that is reflected from a reference region, depending on therequirements of the method; alternatively on the basis of light that isreflected from a reference wafer with good surface characteristics or asurface section thereof; from an illuminated surface area (SAW) of thereference wafer or a semiconductor substrate 21 thereof in a stepperillumination step; or from a single die or a section thereof of areference wafer or the semiconductor substrate 21 thereof. Thecalculation is done in the aforementioned manner, in particular by usingidentical or comparable illumination conditions and/or a smoothing step.The second frequency distribution can be stored in a storage medium.

The position of the center of gravity (reference center of gravity) canalso be calculated for the second frequency distribution in theaforementioned manner.

If the position of the center of gravity of the frequency distribution12 and the reference frequency distribution 12 are not identical, thisresults in a color shift of the light reflected by the semiconductorsubstrate 21 in comparison to the light reflected by the referenceregion. This color shifts is determined by block 9 by the differencegenerated between the two centers of gravity.

Depending on the reference region used and the imaging area used tocalculate the frequency distribution 12, this color shift can be used insubordinate block 10 to determine local color defects or in subordinateblock 11 to detect global color defects. Local color defects which may,for example, be elicited by localized bulging of the thin layer becauseof a dust speck, may, for example, be detected by comparing thefrequency distributions of two local surface areas of one and the samewafer, or by the appearance of an additional peak in the frequencydistribution 12 of the semiconductor substrate 21 being tested. Globalcolor defects, on the other hand, lead to a systematic color shift ofthe peak or of all peaks in the semiconductor substrate 21 that is to betested in comparison to a reference wafer. Systematic color shifts mayoccur both as a result of changed layer thicknesses or missing layers,or in cases of incorrect layers through the use of an incorrect reticle.

Alternatively or additionally, the calculated frequency distribution 12for the semiconductor substrate 21 to be tested can also be shifted fromblock 6 in such a way that its center of gravity is congruent with thereference center of gravity. The shifted frequency distribution 12 inblock 7 is then compared with the second frequency distribution(reference two-dimensional histogram). For this purpose, both frequencydistributions can be overlaid on each other in block 7. In the case oflocal color shifting, an additional peak, for example, can occur in thefrequency distribution 12 of the semiconductor substrate 21 to be testedafter overlaying, which can lead to two peaks of unequal height in theoverlaid total frequency distribution 12, which is comparable to thedistribution 12 according to FIG. 5.

Alternatively, the frequency distributions in block 7 can also besubtracted from each other. In addition, the remaining difference can beamplified by multiplication by a predetermined factor. Small differencesin color distribution 12 can be detected by this means. This isschematically depicted in FIG. 5, wherein the resulting frequencydistribution 12 exhibits a diagonally sloping shoulder 20 and two peaks17, 18.

Obviously, i moments can be calculated for the frequency distributionsand compared with each other, whereby i is a whole number and i≧1.Further information about the color variances on the semiconductorsubstrate 21 can be obtained by this means.

The aforementioned reference regions that can be used to calculate thereference frequency distribution 12 can also be arranged in a presetgeometrical configuration on the reference wafer or on the semiconductorsubstrate 21 to be tested. For example, inherent inhomogeneities in thethickness of the thin applied layer can occur during a processing step.For example, when spin-coating a photoresist layer, the thickness of theapplied photoresist layer may be radially dependent. In such cases, itcan be advantageous to use a ring-shaped area of a reference wafer or ofa semiconductor substrate 21 as the reference region. The size andgeometric form of the reference region may also be varied, according tothe invention.

In cases of deviation of the frequency distribution 12 from thereference frequency distribution 12, a defocusing error duringillumination can also be presumed. Other structures are created on thesemiconductor substrate as a result of defocused illumination, whichexpress themselves in a different frequency distribution 12.

The aforementioned method is in principle suitable for evaluating imagesquickly and completely automatically. As a result, the method may becombined with a method to produce structured semiconductor substrates21, wherein the surface of the semiconductor substrate 21 is testedbetween two processing steps such that if the color shift or thedifferences in the color distribution for the semiconductor substrate 21to be tested are determined to exceed a preset threshold, measures canbe taken to ensure uniformly high quality in semiconductor production.

These measures may, for example, consist in that the semiconductorsubstrate that was just tested, or parts thereof that have been found tobe defective, may be discarded in a subsequent processing step, ortherein that the defective thin layer can be removed from the surface ofthe semiconductor substrate 21 by, for example, rinsing away therecently applied photoresist layer and applying a fresh thin layerthereto in the subsequent processing step, and then testing same untilthe applied layer is found to meet quality requirements. Such aprocedure is known as “after development inspection” (ADI).

For this purpose, an alarm signal, variable, or the like can be createdin case a predetermined threshold value is exceeded, which is thenrelayed to the CPU in the semiconductor production line, initiating theaforementioned means in the process according to the invention.

Other causes of production inhomogeneities in semiconductor componentscan be detected with the help of the method according to the invention.For example, the inventor determined that defocusing during stepperillumination leads to a change in the color distribution of a single SAWas a result of a differently structured surface coating on thesemiconductor substrate 21. Defocusing in a single SAW during stepperillumination is comparatively costly to detect in the state of the art.

The defects that occur can be systematically distinguished as followswith the help of the method according to the invention. Said method canbe used to sort individual semiconductor components or dies. Systematicglobal processing defects can, for example, be differentiated fromlocalized defects. This is because if inadequately thin layers areapplied over the entire surface of a wafer as a result of a systematicprocessing error, said systematic processing error will express itselfas a color shift in all tested dies in comparison to the frequencydistribution 12 of a reference wafer with known (good) surfacecharacteristics, whereas the comparison of the frequency distributionsof individual dies of one in the same wafer do not allow one to inferany significant color shift.

If a single stepper illumination area (SAW) is coated differentlycompared with all other areas of the same wafer, this will expressitself in deviations in the corresponding SAW in comparison both fromthe reference histogram and from the frequency distributions of allother SAWs on the same wafer. To determine whether processing errors arelimited to individual SAWs, it is therefore advantageous if thefrequency distribution 12 is in each case calculated for an entire SAWfor the wafer that is to be tested.

If even defects involving thin coatings on individual dies or substratesare to be detected with the help of the method according to theinvention, it is advantageous if the frequency distribution 12 is ineach case calculated for the die or wafer that is to be tested or for asubarea thereof.

By a suitable choice of the image area used to calculate the frequencydistribution 12 of the wafer that is to be tested, as well as for thereference region used to calculate the reference histogram, the mostvaried requirements of semiconductor production can be flexiblyimplemented according to the invention.

Whereas the aforementioned image sensor detects intensities at varyingwavelengths in the visible spectral region, the present invention is notlimited thereto. In principle, intensities can be detected partially orcompletely in non-visible spectral regions as well. For example, theimage sensor can determine intensities in the infrared, near-infrared,and/or ultraviolet spectral regions. Suitable imaging optics andelements that can be used in the spectral regions will be clear to aperson skilled in the art examining this patent application, and theythus require no further explanation.

The method according to the invention can be realized with the help ofsoftware or a computer program with program code to implement theaforementioned processing steps, which are carried out when the softwareor computer program is implemented on a computer or other suitable dataprocessor, for example on a microprocessor. The software or computerprogram can be stored on a storage medium such as a ROM, EPROM, EEPROM,CD-ROM, DVD, magnetic tape storage, or the like. Suitable hardwarecomponents to carry out the aforementioned computation will be clear toa person skilled in the art examining this description, and may comprisemicroprocessors, ASICs, or specialty signal processors.

FIG. 6 shows a schematic of a wafer 21 that is positioned on a scanningtable 22, and whose surface is captured by a camera 1 (image sensor).Either the entire surface or only a part of the surface of the wafer 21may be captured. In the latter case, the surface is scanned if theentire surface is to be examined. In order to generate a relativemovement between the scanning table 22 and the camera 1, an x-y scanningtable 22 is used that can be moved along the x and y coordinates. Saidcamera 1 is installed securely opposite the scanning table. Obviously,the opposite can also occur, in which the scanning table 22 is securelyinstalled, and the camera 1 is moved over the wafer 21 for the purposeof imaging. A combination in which the camera 1 moves in one directionand the scanning table 22 moves perpendicularly thereto is alsopossible.

The wafer is illuminated with an illumination device 23 that illuminatesat least parts of the wafer surface, e.g., an aforementioned SAW. Inthis case, the illumination is concentrated on the SAW. In addition, astrobe light can be pulsed to capture images on the fly, in which casethe scanning table 22 or the camera 1 are moved without stopping forimaging. This enables a large wafer throughput. Obviously, the relativemovement between the scanning table 22 and a camera 1 can be stoppedduring each imaging, and the wafer 21 can also be illuminated over itsentire surface. The scanning table 22, the camera 1, and theillumination device 23 are controlled by a control unit 24. The imagescan be stored in a computer 25 and processed there as needed.

1. Method for the optical inspection of the surface of a semiconductor(2 1) in which method an image is captured of the surface of asemiconductor substrate (21) that comprises a plurality of pixels havingat least three associated intensities of varying wavelengths that aredesignated as color values; a frequency distribution (12) is calculatedfrom the color values by transformation in a color range that is spannedby one intensity and by color coordinates (U, V), and whose pixels havethe same color coordinate values (u, v); and the frequency distribution(12) calculated in this manner is used for comparison with a secondcorrespondingly calculated frequency distribution or a variable derivedtherefrom.
 2. Method according to claim 1, wherein the color values aredetected in the ultraviolet, visible, and/or infrared wavelength range.3. Method according to claim 1, wherein the color values are transformedin the color range by means of linear transformation, and the frequencydistribution (12) is calculated by adding up the frequency of theoccurrence of pixels having the same color coordinate values in thecolor range, while neglecting the intensity of the pixels.
 4. Methodaccording to claim 1, wherein the color values of a RGB color range aretransformed into a YUV color range in the presence of visiblewavelengths, whereby Y corresponds to the light intensity orluminescence of the pixels, and Y is not considered in calculating thefrequency distribution (12).
 5. Method according to claim 1, wherein thefrequency distribution (12) that is calculated in the color range issmoothed.
 6. Method according to claim 1, wherein macroscopic imagesthat have alternatively at least one semiconductor substrate (21)comprised of a plurality of semiconductor components or dies, or of atleast one surface area (SAW) of the semiconductor substrate (21) that isilluminated in a stepper illumination step, or of a single semiconductorcomponent or die, or of a subarea thereof, are used to calculate thefrequency distribution (12).
 7. Method according to claim 1, wherein thesecond frequency distribution is calculated on the basis of at least oneimage of surface areas that exhibit a given geometric arrangement on awafer, and are in particular radially distributed.
 8. Method accordingto claim 1, wherein a center of gravity is calculated for comparisonfrom the calculated frequency distribution (12), and the position of thecenter of gravity is compared with the position of the center of gravityof the second frequency distribution to detect a color shift for thesemiconductor substrate (21).
 9. Method according to claim 1, whereinfor the comparison the calculated frequency distribution (12) issubtracted from the second frequency distribution to detect differencesin the color distribution for the semiconductor substrate (21). 10.Method according to claim 8, wherein an alarm signal is generated whenthe detected color shift or the detected differences in colordistribution exceed a preset threshold.
 11. Method according to claim 1,wherein for the was will it you'll comparison an i moment of thecalculated frequency distribution (12) is compared with an i moment ofthe second frequency distribution, whereby i is a whole number and i≧1.12. Method according to claim 1, wherein the second frequencydistribution is based on at least one image of a reference wafer and/orof a wafer that is to be inspected.
 13. Method to produce a structuredsemiconductor substrate (21), by which method the surface of thesemiconductor substrate (21) is coated with a thin layer, particularly aphotoresist layer, and the processing steps are implemented according toclaim 8 in order to detect color variances on the surface of thesemiconductor substrate (21).
 14. Method to produce a structuredsemiconductor substrate (21), by which method the surface of thesemiconductor substrate (21) is coated with a thin layer, particularly aphotoresist layer; and the processing steps are implemented according toclaim 10 in order to detect color variances on the surface of thesemiconductor substrate (21); whereby when the alarm signal is given,the semiconductor substrate (21) or subareas thereof are discarded in asubsequent processing step, or the surface of the semiconductorsubstrate (21 ) is freed of the thin layer so that it can be recoatedand re-inspected before the subsequent processing step.
 15. Device forthe optical inspection of the surface of a semiconductor substrate (21);comprising an image sensor (1) to capture an image of the surface of thesemiconductor substrate (21) that comprises a plurality of pixels eachwith at least three associated intensities of varying wavelengths thatare designated as color values; a computational device used to calculatefrom the color values in a color range that spans one intensity andcolor coordinates (U, V) a frequency distribution (12) of pixels havingthe same color coordinate values (u, v); and a means of comparison touse the frequency distribution (12) that is calculated in this mannerfor a comparison with a second correspondingly calculated frequencydistribution, or a variable derived therefrom.
 16. Device according toclaim 15, wherein the image sensor (1) captures the color values in theultraviolet, visible, and/or infrared wavelength range.
 17. Currentlyamended Device according to claim 15, wherein the computational deviceis programmed such that the color values can be transformed by means oflinear transformation in the color range, and the frequency distribution(12) can be calculated by adding the frequency of the occurrence ofpixels having the same color coordinate values in the color range, whileneglecting the intensity of the pixels.
 18. Device according to claims15, wherein the computational device is programmed such that the colorvalues of an RGB color range can be transformed into a YUV color rangeat visible wavelengths, whereby Y corresponds to the light intensity orluminescence of the pixels, and Y need not be considered in calculatingthe frequency distribution (12).
 19. Device according to claims 15,additionally comprising a filter that is used to smooth the calculatedfrequency distribution (12) in the color range.
 20. Device according toclaims 15 in which the computational device is programmed such thatcalculation of the frequency distributions alternatively uses asemiconductor substrate (21) comprising at least one plurality ofsemiconductor components or dies; or at least one surface area (SAW) ofthe semiconductor substrate (21) that is illuminated in a stepperillumination step; or an individual semiconductor component or die; or asubarea thereof.
 21. Device according to claims 15, wherein thecomputational device is programmed such that the second frequencydistribution is calculated based on at least one image of surface areasthat exhibit a given geometric arrangement on a wafer, and are inparticular radially distributed.
 22. Device according to claims 15,wherein the means of comparison is programmed such that it can calculatea center of gravity from the calculated frequency distribution (12) andcompare the position of the center of gravity with the position of thecenter of gravity of the second frequency distribution in order todetect a color shift for the semiconductor substrate (21).
 23. Deviceaccording to claims 15, wherein the means of comparison is programmedsuch that it can subtract the calculated frequency distribution (12)from the second frequency distribution in order to detect differences incolor distribution for the semiconductor substrate (21);
 24. Deviceaccording to claim 22, wherein the means of comparison is programmedsuch that an alarm signal is generated when the detected color shift orthe detected differences in color distribution for the semiconductorsubstrate (21) exceed a given threshold.
 25. Device according to claims15, wherein the means of comparison is programmed such that thecomparison of an i moment of the calculated frequency distribution (12)can be compared with an i moment of the second frequency distribution,whereby i is a whole number and i≧1.
 26. Device for producing astructured semiconductor substrate (21), comprising a coating device tocoat the surface of the semiconductor substrate (21) with a thin layer,particularly with a photoresist layer; and the device according to claim22 is programmed to detect color variances on the surface of thesemiconductor substrate (21).
 27. Device for producing a structuredsemiconductor substrate (21), comprising a coating device to coat thesurface of the semiconductor substrate (21) with a thin layer,particularly with a photoresist layer; and the device according to claim24 is programmed to detect color variances on the surface of thesemiconductor substrate (21); whereby the device is programmed such thatwhen the alarm signal is given, the semiconductor substrate (21) orsubareas thereof can be discarded in a subsequent processing step, orthe surface of the semiconductor substrate (21) can be freed of the thinlayer and recoated before the subsequent processing step.
 28. Software,in particular a computer program, comprising program code to implementall steps according to claims 1 when the software or computer program isrun on a computer or a data processor.
 29. Software, in particular acomputer program with program code according to claim 28 that can bestored on a data storage medium readable by a computer.